A revision of Obruchevia (Psammosteida: Heterostraci) and a description of a new obrucheviid from the Late Devonian of the Canadian Artic

Psammosteids are the youngest heterostracans, surviving until the end of the Frasnian in western Europe where their faunal succession is well known. Recent collections made by the 1999-2002 Nunavut Paleontological Expeditions from the Devonian clastic wedge across Melville, Bathurst, Devon, and Ellesmere islands now show a similar psammosteid faunal succession in the Canadian Arctic. Some very thick psammosteid plates from southern Ellesmere lack dentine tubercles but do have an increased amount of the hard tissue pleromin infilling the spongy aspidin at the surface. This feature is otherwise known only in the psammosteid Obruchevia, described from the Lovat ́ River, Novgorod District, northwestern Russia. The dorsal plates of Obruchevia are large, notably thick, and cardiform and appear to have grown by the addition of lateral flanges that developed from the lower surface of the margins. The surface is ornamented with radial furrows and pits. The branchial plates have a vertically directed lateral margin that would have functioned as a runner. Previously undescribed specimens from the Lovat ́ River, housed in the collections of the Natural History Museum of Latvia, Riga, confirm the structure of the branchial plates and show that the ventral plate, not known before in Obruchevia, had a deep posterior notch similar to that found in Schizosteus, Pycnolepis, Pycnosteus, Ganosteus, and Tartuosteus. The almost complete specimens of the branchial plates from the Palaeontological Institute, Russian Academy of Sciences, Moscow, allow this plate to be more fully described. Although the obrucheviid from the Canadian Arctic is incomplete and shows an ornament of large elongated blisters and irregular ridges rather than pits and grooves, it also possessed a ventral plate with a well-developed posterior notch. In addition a well-developed dorsal sensory canal system is present as open surface grooves, an unusual feature in psammosteids. This species is clearly related to Obruchevia within the Obrucheviidae.